To improve upon the accuracy of positioning and navigation data obtainable using a Global Navigation Satellite System (GNSS) receiver, many techniques are employed. These include techniques such as differential corrections and Real Time Kinematics. A common feature of many of these techniques involves sending supplemental information to a GNSS receiver. This supplemental information is in addition to the information, such as broadcast orbits, that is received over-the-air via transmissions from GNSS satellites.
The supplemental information, referred to herein as “refinement information,” is used at the GNSS receiver to refine the positioning and navigation data calculated by the GNSS receiver. Refinement information may include information such as: differential corrections (often referred to as correctors) calculated at a location other than the GNSS receiver; precise satellite orbits which are more exact than the contemporaneously valid over-the-air satellite orbits received from GNSS satellites; and raw measurable information (compressed or uncompressed) supplied from a differential or real-time kinematics base station or other source.
In addition to refinement information, handshaking information in the form of an 8-bit Issue of Data Ephemeris (IODE) handle is also often sent from a GNSS base station to a GNSS receiver. At the GNSS receiver, a received IODE handle is compared to an IODE handle associated with a broadcast orbit in use at the GNSS receiver to ensure that the same orbit is being used to perform calculations with refinement information as was used to originally calculate the refinement information. This handshaking, with respect to differential techniques, ensures that orbits are synchronized and thus allows the concept of common mode error rejection to be employed to cancel out certain errors that are common between a GNSS base station and a GNSS receiver. For example, a differential calculation performed using correctors received by a GNSS receiver will generate more accurate positioning and navigation data when the GNSS receiver uses the same orbit as used in originally calculating correctors. This is the case even if the orbit used at the GNSS receiver is more accurate than the orbit used to originally calculate the correctors. With respect to Global Positioning System (GPS) satellites, each GPS satellite transmits a new broadcast orbit for itself every two hours. Thus, in geographically separated GNSS base stations and GNSS receivers there exists a short window, after the beginning of the transmission of a new broadcast orbit, when orbit handshaking between a base station and a receiver takes on an increased importance. This is because either the base station or receiver may have received the new orbit before the other has received it.
To send a precise position or orbit for a single satellite could require 200 or more bits of information. To send an IODE for a single satellite requires 8-bits of information. Often there are as many as 12 visible GPS satellites for which refinement information or orbit handshaking information is supplied. This refinement information and orbit handshaking information may be supplied frequently, such as once per second, for each satellite. This can require a substantial number of bits. Moreover, the bits required to transmit such information increases even further if the information is supplied for other navigation satellites, such as, for example, the GLObal NAvigation Satellite System (GLONASS) satellites, future Galileo (European) satellites, and/or future Compass (Chinese) satellites.
If bandwidth were unlimited, this would not be a problem. However, bandwidth between a GNSS base station and a GNSS receiver is often limited by factors such as costs or transmission rates. For example, in some cases, such as over a cellular connection between a GNSS base station and a GNSS receiver, a user must pay a fee for the total amount of data transmitted or time over which data is transmitted. Thus additional data incurs additional fees. In other cases, transmission distances and power restrictions on transmitters in frequency bands used to support transmission between a GNSS base station and a GNSS receiver often limit transmission rates to very low speeds which are often in the range of 100 to 9600 bits/second.